IBM PC compatible
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IBM PC compatible computers are those generally similar to the original IBM PC, XT, and AT. Such computers used to be referred to as PC clones, or IBM clones since they almost exactly duplicated all the significant features of the PC architecture, facilitated by various manufacturers' ability to legally reverse engineer the BIOS through clean room design. Columbia Data Products built the first clone of an IBM personal computer through a clean room implementation of its BIOS. Many early IBM PC compatibles used the same computer bus as the original PC and AT models. The IBM AT compatible bus was later named the ISA bus by manufacturers of compatible computers.
The term "IBM PC compatible" became exclusively historical once IBM stopped manufacturing personal computers.
Descendants of the IBM PC compatibles make up the majority of microcomputers on the market today, although interoperability with the bus structure and peripherals of the original PC architecture may be limited or non-existent.
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[edit] Origins
The origins of this platform came with the decision by IBM in 1980 to market a low-cost single-user computer as quickly as possible in response to Apple Computer's success in the burgeoning market. On 12 August 1981, the first IBM PC went on sale. There were three operating systems (OS) available for it but the most popular and least expensive was PC DOS, a version of MS DOS licensed from Microsoft. In a crucial concession, IBM's agreement allowed Microsoft to sell its own version, MS-DOS, for non-IBM platforms. The only proprietary component of the PC was the BIOS (Basic Input/Output System).
A number of computers based on the 8086/8088 processors, but with different architecture to the PC, and which ran under their own versions of MS-DOS and CP/M-86, were manufactured during this period. However, software which addressed the hardware directly instead of making standard calls to MS-DOS was faster. This was particularly relevant to games. The IBM PC was the only machine sold in high enough volumes to justify writing software specifically for it, and this encouraged other manufacturers to produce machines which could use the same programs, expansion cards and peripherals as the PC. The 808x computer marketplace rapidly excluded all machines which were not functionally very similar to the PC. The 640kB limit on "conventional" system memory available to MS-DOS is a legacy of that period; other non-clone machines did not have this limit.
The original "clones" of the IBM Personal Computer were created without IBM's participation or approval. Columbia closely modeled the IBM PC and produced the first "compatible" PC (i.e., more or less compatible to the IBM PC standard) in June 1982 closely followed by Eagle Computer. Compaq Computer Corp. announced its first IBM PC compatible a few months later in November 1982—the Compaq Portable. The Compaq was the first sewing machine-sized portable computer that was essentially 100% PC-compatible. The company could not directly copy the BIOS as a result of the court decision in Apple v. Franklin, but it could reverse-engineer the IBM BIOS and then write its own BIOS using clean room design. Compaq became a very successful PC manufacturer, but was bought out by Hewlett-Packard in 2002.
[edit] Compatibility issues
- See also: Influence of the IBM-PC on the PC market
Simultaneously, many manufacturers such as Xerox, HP, Digital, Sanyo, Texas Instruments, Tulip, Wang and Olivetti introduced personal computers that were MS DOS compatible, but not completely software- or hardware-compatible with the IBM PC. Microsoft's intention, and the mindset of the industry from 1981 to as late as the mid-1980s, was that application writers would write to the application programming interfaces (APIs) in MS-DOS, and in some cases to the firmware BIOS, and that this level of interface would form what would now be called a hardware abstraction layer. Each computer would have its own Original Equipment Manufacturer (OEM) version of MS-DOS, customized to its hardware. Any piece of software written for MS-DOS would run on any MS-DOS computer, regardless of variations in hardware design.
This expectation seemed reasonable in the context of the computer marketplace as it existed then. Until then Microsoft was primarily focused on computer languages such as BASIC. The established small system operating software was CP/M from Digital Research, which was in use both at the hobbyist level and at the more professional end of the microcomputer spectrum. To achieve such widespread use, the OS had to operate across a range of machines from different vendors that had widely varying hardware. Those customers who needed additional applications beyond the starter pack could expect publishers to offer their products in several media formats for a variety of computers.
Microsoft's competing OS was initially targeted to run on a similar varied spectrum of hardware, although all based on the 8086 pro